A geo-fence is a virtual fence bounding a real-world geographic region. This virtual fence can be used as part of a geo-fencing system to detect when a person within the bounded area leaves the bounded area. An example application is monitoring whether a person with dementia (e.g., from Alzheimer's disease) leaves a “safe zone” (e.g., personal house, group home, etc.) defined by the virtual fence. In response to detecting the person has left the bounded area, the geo-fencing system triggers transmission of a signal (e.g., email, text message, smartphone notification, etc.) which indicates the person has left the bounded area.
The “safe zone” can be set up by manually measuring a distance from the base device to a desired location, for one or more desired locations. The greatest distance of the distances for these one or more desired locations is then manually entered into the “safe zone” running on the base device, which creates a circular “safe zone” around the base device with a radius equal to the greatest distance. Unfortunately, not all desired locations are located linearly from the based device, making certain measurements difficult and error prone. Furthermore, a distance measurement made by the base device for a particular location may be different from the manually measured distance for that location, e.g., due to device calibration, signal strength, etc.
A geo-fence based on Received Signal Strength Indicator (RSSI) or Time of Flight (ToF) (or Sine Phase based) can be monitored using a base device located at a center of the geo-fence and a portable device worn or carried by the person being monitored. The distance between the worn device and the base device is measured and compared to a distance between the base device and a perimeter of the virtual fence. RSSI and ToF based systems are based on a self-generated signal and thus are not dependent on coverage of other systems (e.g. GPS, cellular, etc.), and their power dissipation is relatively low making them well-suited for devices that require a battery for the power supply.
Unfortunately, RSSI signal strength is dependent on the structures around it. For example, the strength through a window or door will be different from the strength through a wall, and a metal object (e.g. a car) in the direct neighborhood will also influence the signal strength. As a consequence, using RSSI may result in a non-uniform, conditionally dependent, and/or unreliable distance measurement. ToF is less dependent on these parameters. However, when a person is standing in between the two devices ToF measurement errors can occur because the signal, e.g., via reflection, is stronger than the through the human body. This is also the case of no direct sight where a wall reflection can give measurement errors.
In view of at least the above, there is an unresolved need for another approach for geo-fencing.